1. Field of the Invention
The present invention relates generally to methods and apparatuses for modulation encoding data for either storage on a multi-level recording medium or for transmission across a multi-level communication link. More specifically, the invention relates to a method of encoding data either to be stored on a multi-level optical storage disc which allows more than two potential data states to be stored at each physical location on the optical disc or to be transmitted across a communication link which allows more than two potential data states to be transmitted at any given time. As the number of potential data states stored or transmitted increases, the incidence of errors tends to increase when the maximum resolution of either the data writing/transmitting or data reading/receiving system is approached. The modulation encoding scheme described puts constraints on the data that is stored or transmitted in an attempt to maximize the number of potential data levels that can be read or received without the occurrence of an unacceptable number of errors.
2. Description of the Related Art
Current optical data storage discs store data in the form of marks which are formed on one surface of the disc. The mark is read by focusing a light source on the surface of the disc with the marks and detecting the intensity of reflected light from this surface. The intensity of light is generally converted to a voltage signal. When the light reflects off the disc surface where there is no mark, the light reflected from the disc is one value. For current read-only (ROM) discs, this is a high value since light is reflected from regions where there is no mark. However, when the light is incident on a mark, the light reflected from the disc surface is another value, which is typically a lower value for ROM discs. It is also possible to store more than one bit of information at each mark or symbol location by modulating the reflectivity of the marks. Such a disc is referred to as a multi-level (ML) modulated disc or, because the marks are actually pits in the case of current ROM discs, a pit depth modulated (PDM) disc. Read-only, write-once, and re-writable discs with multi-level reflectivity marks are possible using disc technologies similar to existing optical data storage technologies, such as compact disc (CD) or digital video disc (DVD).
ML or PDM discs are capable of storing more than 1 bit of data at each data storage position on the disc or mark. It should be noted that throughout this specification the terms "mark" and "storage position" are both used to describe a point on the disc where data is stored. Information states are defined for each mark and when a mark is read, then one of I information states is determined based on the mark that is read. Each potential information state may correspond to a symbol output by a modulation encoding scheme that converts data into symbols that are written to a multilevel media or transmitted across a multilevel transmission media. Thus, each mark may store one of I information states and without further coding, the number of bits, n, stored at each mark is n=log.sub.2 I.
Light reflected from one mark also tends to interfere with light reflected from another mark, especially when the marks are smaller than the reading laser spot of the optic stylus. This results in intersymbol interference (ISI). In other words, the signal from and location of one mark, therefore, tends to influence or interfere with the signal that is read from adjacent or neighboring marks. As the areal density of the marks increases, the ISI effect increases. A "modulation transfer function" (MTF) describes the transformation of the detected signal that results from the diffraction of light from neighboring pits. U.S. patent application Ser. No. 08/852,242, now U.S. Pat. No. 5,818,806, titled "Method and Apparatus for Providing Equalization for the Reading of marks on optical Data Storage Media" by Wong et. al., which is herein incorporated by reference for all purposes, describes a method and apparatus for providing equalization for a signal generated by reading a PDM disc. The method described compensates for intersymbol interference.
Even when the improved signal processing techniques described in Wong et. al. are used, it is nevertheless true that as the number of information states I that are stored increases, the more difficult it becomes to distinguish between the I different information states during reading. This increases the incidence of errors made in decoding the information stored in the marks.
FIG. 1 is an illustration of an ideal signal read from a PDM disc that has I=8 different information states. The y-axis indicates the intensity of the reflected light from the disc, and the x-axis indicates the distance, or length, traveled when reading the marks on the disc. Such a disc would be capable of storing 3 bits of information. No noise or intersymbol interference is included in the signal. For a real signal, the levels would include noise that would alter the signal level. Because of the precision of the measurement used to distinguish signal levels or because of the inherent system noise, the chance of error may be unacceptably high. The error rate could be significantly reduced if only two or four information states were possible for each mark instead of eight. However, this would of course decrease the amount of information that could be stored on the disc.
The problem of efficiently eliminating errors in such a situation is complex. Simply reducing the number of information states and thereby increasing the difference in signal output for different information states written to the disc would decrease the error rate, but at the cost of decreased density of data storage. Reducing the number of information states below 8 in the example given above would reduce the storage capacity of the system and would potentially be an inefficient use of the system.
Conventional compact discs utilize a modulation code that facilitates reading information from the disc as well as detecting and eliminating errors. In order to read information from a disc, the reading system must locate the marks, focus a reading laser on the disc surface, maintain accurate tracking of the laser over the tracks of marks, and recover timing information from the marks. Errors tend to occur when the disc is read. The problem of detecting and correcting errors on a PDM disc is different than error detection and correction on a conventional optical disc. Since the number of information states equals 2 in conventional discs, information is stored by modulating the length of the marks. When the signal level of the mark is modulated between more than two information states, the type of errors that occur changes. As a result, conventional modulation codes used in CD or DVD storage do not provide a way to effectively decrease the error rate while maximizing data storage capacity in a PDM environment.
Specific run length limited modulation encoding schemes for storing information on a multilevel optical recording medium are described in the following US Patents:
U.S. Pat. No. 5,657,014 M=7(3,7) Run length Limited Code for Multilevel Data (1997). PA1 U.S. Pat. No. 5,659,310 M=5(0,2) Run length Limited Code for Multilevel Data (1997). PA1 U.S. Pat. No. 5,659,311 M=6(2,4) Run length Limited Code for Multilevel Data (1997). PA1 U.S. Pat. No. 5,663,722 M=10(3,6) Run length Limited Code for Multilevel Data (1997). PA1 U.S. Pat. No. 5,663,723 M=7(1,3) Run length Limited Code for Multilevel Data (1997). PA1 U.S. Pat. No. 5,668,546 M=6(3,6) Run length Limited Code for Multilevel Data (1997). PA1 U.S. Pat. No. 5,670,956 M=5(3,7) Run length Limited Code for Multilevel Data (1997). PA1 U.S. Pat. No. 5,675,330 M=5(4,11) Run length Limited Code for Multilevel Data (1997). PA1 U.S. Pat. No. 5,680,128 M=6(3,8) Run length Limited Code for Multilevel Data (1997). PA1 U.S. Pat. No. 5,682,154 M=4(1,2) Run length Limited Code for Multilevel Data (1997). PA1 U.S. Pat. No. 5,682,155 M=6(4,11) Run length Limited Code for Multilevel Data (1997).
These patents describe multilevel modulation codes that can have a specific number of different levels, or amplitudes, and specific number of lengths, usually expressed as minimum integer and maximum integer length. However, the disclosed codes do not have properties that enable errors to be controlled. What is needed is a multilevel modulation code that includes error control.
Another multi-level modulation code is described in U.S. Pat. No. 5,537,382 Partial Response Coding for a Multilevel Optical Recording Channel (1996). The disclosed code requires a complex coding strategy that involves typical equalization issues faced by partial response codes. It would be useful if an approach could be developed that would provide a simpler decoding strategy.
As described above, if an effective, simple multi-level modulation code with error control could be developed, then the efficient storage of digital information on a multi-level optical disc would be facilitated. In addition, it should also be noted that multiple data levels have also been explored for use in data transmission systems such as those used in communication technologies including cellular telephones, modems, and fiber optic networks. These systems use a variety of encoding schemes including binary coding and quadrature amplitude encoding. In such systems, as in multilevel optical storage systems, an effective multi-level modulation coding scheme that controls errors is needed.